1. Field of the Invention
The present invention relates to a technology ideally used for a thin inner rotor motor for rotatably driving a medium that is used with, for example, a floppy disk drive unit or the like.
2. Description of the Related Art
A disk apparatus, such as a floppy disk unit, has been extensively used for a personal computer, an office computer, a word processor, etc., and has been showing remarkable widespread use. This type of disk apparatus is constructed, for example, as shown in FIG. 10.
The disk apparatus will be outlined in conjunction with FIG. 10. A chassis 101 has a spindle center 102 providing the rotational center of a disk, and is accommodated in an equipment housing (not shown) of, for example, a personal computer or the like. The chassis 101 is formed by a bottomed box that is opened frontward and upward and has a space for accommodating a disk cartridge 103.
At the rear end of the chassis 101, a stepping motor 124 for driving a head carriage, and the head carriage constructed to be moved back and forth by the stepping motor 124 are provided. A first head 130 for reading the information recorded on a disk is retained on the distal end of the head carriage, and a head arm 132 having a second head 131 associated with the first head 130 is attached to the rear upper end such that it may swing through the intermediary of an elastic member. The head arm 132 is urged such that the second head 131 is urged toward the first head 130. The disk apparatus in this example is provided with a cartridge holder 136 that removably holds the disk cartridge 103, and a device for opening and closing the shutter of the disk cartridge 103.
Some disk apparatuses of this type use an inner rotor motor shown in FIGS. 11A and 11B as the motor for rotating a disk in order to achieve a thinner design in recent years.
The inner rotor motor is constructed by a stator 164 that has an annular yoke 161 extending in the circumferential direction and multiple cores 163 which are radially provided on the inner circumferential surface of the yoke 161 and around which coils 162 are wound, and a rotor 166 having an annular magnet 165 which is rotatably provided on the inner circumferential surface of the stator 164 so that it opposes the cores 163. The inner rotor motor further includes a circuit board 168 on which a retaining assembly 170 that incorporates a bearing 169 is mounted, and a rotary shaft 171 for fixing the rotor which is rotatably supported by the retaining assembly 170 on the circuit board 168 through the intermediary of the bearing 169 and which has an axis extending in the vertical direction. The rotor 166 of the inner rotor motor functions as a turntable that has a disk chucking magnet (not shown) and a disk chucking rotary lever (not shown).
In the stator for this type of inner rotor motor, the yoke 161 and the cores 163 are provided such that they surround substantially the whole circumference of the circular rotor 166 except for the portion wherein the heads 130 and 131 move. In order to meet magnetic characteristics requirements, the stator is formed of silicon steel or the like that is more costly than a galvanized steel plate constituting the chassis 101, etc.
For this type of disk apparatus, however, there has been a constant demand for reduced manufacturing cost, and also a high demand for a smaller size and a reduced weight of the disk apparatus. Hence, a demand exists for reducing the areas of the yoke 161 and the cores 163 formed of the costly silicon steel in the stator for the inner rotor motor.
If, however, the areas of the yoke 161 and the cores 163 are reduced to meet the aforesaid demand, then the magnetic interaction relative to the rotor 166 may become uneven in the circumferential direction, adversely affecting the operational stability of a disk.
In addition, another problem exists. When an attempt is made to reduce the size and weight of a floppy disk drive unit or the like, the possibility increases in which the magnetic head for writing and reading magnetic information from and to a medium, such as a floppy disk, is brought excessively close to the magnet of the rotor, and the magnetic fluxes from the magnet of the rotor 166 to the magnetic head adversely affect the writing and reading of the magnetic information.
Furthermore, the magnetic fluxes that enter into the chassis 101 from the magnetic poles of the rotor 166 cause a force to act between the rotor 166 and the chassis 101. This consequently causes an applied thrusting force to push the rotor 166 against the chassis 101, possibly leading to deteriorated rotation of the rotor 166. On the other hand, if the thrusting force for pushing the rotor 166 against the chassis 101 is too weak, then the rotational stability of the rotor 166 deteriorates. Therefore, there has been a demand for controlling the thrusting force to remain within a preferable range.